DrawLine Draw a line from one point to the other with the given color following the drawing flags .
画一条线从一个点到另一个点，用给的颜色跟着指定的标志。
Once the marker is added to the map as an overlay , the drawLine () function is called .
将这个标记作为叠加层添加到地图上之后，调用drawLine（）函数。
drawline